This invention relates generally to copying or printing apparatus, and more particularly, it relates to the heat and pressure fixing of particulate thermoplastic toner by direct contact with a heated fusing member.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto in one of various ways, for example, as by heat and pressure.
In order to affix or fuse electroscopic toner material onto a support member by heat and pressure, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky while simultaneously applying pressure. This action causes the toner to flow to some extent into the fibers or pores of support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy and pressure for fixing toner images onto a support member is old and well known.
One approach to heat and pressure fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll thereby to effect heating of the toner images within the nip. With this conventional type of arrangement, the temperature profile through the nip is somewhat exponential while the pressure profile is symmetrical. A plot of nip pressure versus position in the nip yields a somewhat parabolic shape. A symmetrical pressure profile results in the application of high pressure to toner which is not yet in a molten state. This results in wasted mechanical energy.
As fuser speeds increase, it becomes more and more difficult to obtain adequate nips using roll fusers because the nip width varies approximately as the square of the roll diameter. Thus, for example, doubling the process speed would require double the nip width which, in turn, would increase the fuser and pressure roll diameters by a factor of four. In addition, larger rolls require higher loads and produce an inferior release geometry. The foregoing drawbacks do not apply to belt fusers. Thus, belt fusers of the prior art have been provided with larger nip areas in order to allow faster fusing speeds. However, all known prior devices inherently waste mechanical energy due to their symmetrical pressure profiles.
Belt fusers are known in the prior art. For example, U.S. Pat. Nos. 4,563,073 and 4,565,439 each disclose a heat and pressure fusing apparatus for fixing toner images. The fusing apparatus is characterized by the separation of the heat and pressure functions such that the heat and pressure are effected at different locations on a thin flexible belt forming the toner contacting surface. A pressure roll cooperates with a stationary mandrel to form a nip through which the belt and copy substrate pass simultaneously. The belt is heated such that by the time it passes through the nip its temperature together with the applied pressure is sufficient for fusing the toner images passing therethrough. A release agent management (RAM) system comprising low mass donor and metering rolls, one of which is in contact with the belt, applies silicone oil to the belt without unacceptably reducing the fusing capability of the belt.